1. Grayjay can cast to an FCast receiver
2. There are some terminal clients and a nautilus plugin, but we don't provide binaries (yet). These can be used to cast local media and remote media https://gitlab.com/futo-org/fcast/-/tree/master/client
3. Someone made a yt-dlp client that can cast to FCast: https://git.sr.ht/~shironeko/fcast/tree/yt-dlp/item/clients/...
4. Cloudfstream supports it: https://github.com/recloudstream/cloudstream/blob/master/app...
https://gitlab.com/futo-org/fcast/-/wikis/Protocol-version-1
As you can see the 1.0 protocol is very lean and fits on an A4.
If you want to have an audio-only ESP32 speaker I don't see why it wouldn't be possible with the FCast protocol.
Briefly looked at SoundCloud script js, for the moment it seems to be written to pull frontpage with basic API implementation, correct me if I'm wrong.